北京平原区南部PGZ01孔第四纪地层划分及其环境意义

Quaternary stratigraphic division and its environmental significance of Borehole PGZ01 in southern Beijing plain area

对北京平原区南部大兴榆垡镇420 m深的PGZ01孔岩心进行了岩石地层、磁性地层、14C测年、介形类组合和孢粉组合研究。结果表明:(1)Matuyama(松山)/Gauss(高斯)和Brunhes(布容)/Matuyama(松山)极性界线分别位于280 m和78.35 m处,钻孔底部已经进入Gauss正极性带;取280 m处Matuyama/Gauss极性界线作为第四系下限,显示可能的永定河泥石流—冲积扇在第四纪之前即到达了北京平原南部,但是华北平原上新统标志岩石地层单元——厚层棕红色黏土在Olduvai(奥尔都维)极性亚带之上即已出现。(2)结合AMS14C测年,尝试将约60 m以上地层的氧化—还原状况,与对古季风影响反映灵敏的沿海平原钻孔同样深度地层进行对比,继而确定上更新统和全新统的底界深度分别位于56.60 m和18.95 m。(3)MIS5(海洋同位素阶段)以来的特征沉积物,与沿海平原钻孔地层有很好的可比性,同时也有8~4 ka期间史前大洪水的影响。本研究为廊(坊)固(安)凹陷西北边缘区第四纪地层划分对比、古地理环境演化、活动断裂以及区域差异构造沉降研究,提供了重要的年代地层格架,对理解晚更新世以来古季风变化的环境效应有重要的启迪。

Based on the core materials collected from the Borehole PGZ01,which is located at Yufa Township of Daxing district in southern Beijing plain area and has a depth of 420 m,the analyses of lithostrotigraphy,magnetostratigraphy,AMS14 C dating,Ostracoda and sporopollen assemblages were carried out. The results show that：（ 1） The boundaries of Matuyama/Gauss polarity chron and Brunhes/Matuyama polarity chron appear at the depth of 280 m and 78. 35 m,respectively. It suggests that the bottom strata of this borehole correspond to Gauss normal polarity zone at the bottom. Considering that the boundary of Matuyama/Gauss polarity chron at the depth of 280 m is the lower boundary of Quaternary system. It suggests that the debris flow and alluvial fan or possible palaeo-Yongding River had arrived at the southern part of Beijing plain area before Quaternary. Moreover,thick-layer brownish red clay which is considered as the upper mark lithostratigraphic unit of the Pliocene occurred at the Olduvai subzone.（ 2） Integrated with the AMS14 C dating,the present study tentatively correlate the stratal oxidation-reduction status above the depth of 60 m to correlate to cores from palaeomonsoonal-sensitive coastal plain environment, the boundary of the Upper Pleistocene and Holocene was determined to be at 56. 60 m and 18. 95 m,respectively.（ 3） The characteristic sediments deposited since MIS5 have a good correlation with core sediments from coastal plain environment,and show evidence of the influence of Prehistoric Flood of 8-4 ka. The present paper provided important age control for subdividion and correlation of Quaternary stratigraphy,construction of palaeogeography,fault activities and tectonic history of the westnorthern margin region of Lang（ fang）-Gu（ an） Depression, and understanding of the environmental effect of palaeomonsoon change since the Late Pleistocene.